The present invention generally relates to a method for manufacturing a semiconductor device, and more specifically, to a method of subjecting a semiconductor substrate to thermal treatment at a temperature ranging from 770 to 830° C. to fix channel ions then forming a HTO(High Temperature Oxide) film. The method thereby prevents a threshold voltage of a gate from changing due to diffusion of channel ions.
Due to high-integration of semiconductor devices, a recess gate is formed to regulate a threshold voltage of the gate. A local gate portion of a semiconductor substrate has an active region where a channel-ion-implanting portion is partially etched to form a recess portion. In order to form a hard mask pattern for forming a recess portion, an oxide film and an anti-reflection film are used. The oxide film includes a PE-TEOS film or a TEOS film that has a relatively rapid deposition speed. The PE-TEO film or the TEOS film is formed at a temperature of 700° C. The temperature ranging from 680 to 700° C. is the optimum temperature for diffusing impurities distributed in the channel-ion-implanting portion. This phenomenon is called TED (Thermal Enhancement Diffusion) which causes a threshold voltage of the gate to change in a gate-forming-process. The threshold voltage is reduced in case of NMOS. The threshold voltage is increased in case of PMOS. This change of the threshold voltage degrades electrical characteristics of high-integrated semiconductor devices.